Hundreds of people with Freeview are complaining after losing some channels when they tried to retune their set-top boxes and TVs.
The service is being upgraded to give more homes channel Five, but 460,000 households are expected to lose access to ITV3 and ITV4 instead.
Maurice Short from Brighton is "very annoyed" because he has lost ITV3 but gained channel Five.
A Freeview spokesperson said viewers with problems should try again.
A statement said: "The majority of people find retuning straightforward but if viewers are experiencing problems after retuning, we would suggest trying again."
There is also a postcode checker available online for people to check what channels they should expect to receive.
Hamilton White from Lochaber in Scotland said he has lost ITV3 and has attempted to contact Freeview, "who are not accepting calls due to 'high call volumes'".
Freeview said the helpline was receiving about 200 calls a minute and "most" people were getting through to an agent but if one is not available, then a recording of basic instructions is being played out.
Freeview admitted that the website has been crashing because of high demand, but it was working "intermittently".
Armand Derri from Edmonton in London started off with over 30 channels but now claims to have "lost nearly all my channels now".
People with Freeview were advised to retune their TV sets, set-top boxes and digital recorders from lunchtime on Wednesday.
It is thought about 20,000 older set-top boxes will cease to work.
The boxes which will no longer work are: the Daewoo models DS608P and SV900, Labgear DTT100, Triax DVB2000T and Bush IDVCR01.
The technical changes behind the upgrade will eventually enable high-definition (HD) television to be broadcast on Freeview. HD is already available on the free-to-air satellite service Freesat.
"These are significant and necessary changes which will immediately bring Freeview viewers new channels, introduce new homes to existing channels and prepare the platform for the future availability of Freeview HD," said Ilse Howling, managing director of Freeview.
All Freeview TV sets, set-top boxes and digital recorders, including TopUp TV and BT Vision boxes, will need to be retuned from lunchtime on 30 September.
The upgrade will affect around 18 million homes, and it thought that about 25 million televisions and set-top boxes will need retuning.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.